docs: add OpenBao secret storage guide

[ajile-in]

Summary

Adds documentation for the optional OpenBao secret storage feature shipped in the platform PR (credebl/platform#1679). The CREDEBL Platform can now source email (Resend/SendGrid/SMTP) and AWS S3 credentials from an OpenBao server instead of plain .env variables.

Changes

• New page getting-started/local-deployment/openbao.md — user-facing setup guide:
  • Run the server via docker compose -f docker-compose.openbao.yml up -d
  • One-time idempotent provisioning via ./openbao-init.sh
  • .env configuration (ENABLE_BAO, SECRETS_PROVIDER, BAO_* variables)
  • Secret path → credentials table
  • Startup injection + runtime TTL-cached secret loading
  • How to disable (fall back to env vars)
• SUMMARY.md — OpenBao page nested under Local Deployment → Platform
• platform.md — optional OpenBao hints in the AWS S3 and SendGrid prerequisite sections
• local-deployment/README.md — one-line mention in the Platform component
• service-setup.md — pointer to the new guide in the Environment Variables section

Notes

• OpenBao storage is optional and disabled by default; existing .env-based setup is unchanged.
• No changes to cloud-deployment pages (OpenBao is not wired into the install repo yet).